Lubricating oils are extracted using N-methyl pyrrolidone so as to have their aromatics content reduced. This extraction is usually performed at temperatures in the range of about 70.degree. F. to about 300.degree. F. Any hydrocarbon feed that has an initial boiling point at least about 100.degree. F. to 150.degree. F. above the boiling point of pure NMP solvent (399.degree. F.) is a suitable lube oil stock for extraction using NMP. Lube oil feeds comprise petroleum fractions having an initial boiling point of above about 500.degree. F. These fractions include deasphalted oils and/or distillate lube oil fractions boiling within the range of about 600.degree. F. and 1,050.degree. F. (at atmospheric pressure) and contain between about 5% and about 70% (by weight) of polar and aromatic compounds, such as substituted benzenes, naphthalenes, anthracenes and phenanthracenes, characterized by having a carbon content typically in the range of C.sub.15 -C.sub.50. Non-limiting examples of useful feedstocks include crude oil distillates and deasphalted resids, those fractions of catalytically cracked cycle oils, coker distillates and/or thermally cracked oils boiling above about 600.degree. F. and the like. These fractions may be derived from petroleum crude oils, shale oils, tar sand oils and the like. These fractions may come from any source, such as the paraffinic crudes obtained from Aramco, Kuwait, The Panhandle, North Louisiana, etc., naphthenic crudes, such as Tia Juana and Coastal crudes, etc., as well as the relatively heavy feedstocks, such as Bright Stocks having a boiling range of 1,050.degree. F.+ and synthetic feedstocks derived from Athabasca Tar Sands, etc.
Solvent extracting lube oil fractions using NMP produces raffinate and extract phases containing NMP. Because lube oil extraction with NMP is normally performed at temperatures below about 300.degree. F. it is necessary to heat the NMP containing phases to higher temperatures in order to separate the solvent therefrom if thermal recovery means are used. Generally, this temperature must be at least 400.degree. F. in order to separate the oil and solvent, using either flash evaporation and/or distillation, because NMP boils at about 395.degree. F. or higher, depending on its purity. This minimum thermal separation temperature is readily achieved by heating the solvent containing phases in heat exchangers, such as direct fired tube furnaces, and then passing the hot raffinate and/or extract solutions to flash towers, distillation towers or combinations thereof. The bulk temperature of the hot, solventcontaining oil often exceeds 500.degree. F. and portions thereof may even exceed temperatures in excess of 700.degree. F. Material contained in the thin boundary layer film inside the furnace tubes may be heated to temperatures of 800.degree. F. or more, particularly in the radiant section of the furnace.
It has been found that NMP significantly decomposes when heated to temperatures at or above 700.degree. F.
Beyond this, however, even NMP recovered at 600.degree. F. and higher contains a significant quantity of contaminants which are detrimental to the structural integrity of the recovery circuit. At the high temperatures encountered in the NMP recovery circuit of a lube oil extraction process chemical reactions occur which are not observed in other extraction solvent recovery schemes. For example, in solvent recovery schemes in aromatics extraction processes practiced in the chemical industry the solvent, since it boils at a temperature higher than the extract/oil, is recovered at a lower temperature in the 150.degree. F. to 350.degree. F. range and, therefore, thermal decomposition and chemical conversion of contaminants are minimized, if not totally avoided.
In the recovery of NMP in lube oil extraction processes use of the high recovery temperatures (600.degree. F.) result in the conversion of dissolved salts, such as sodium chloride, to hydrochloric acid, iron sulfide to hydrogen sulfide. Further, lube distillate feeds contain organo-sulfur and organo-nitrogen compounds which are known to degrade to form hydrogen sulfide and ammonia at the temperatures involved. Lube distillate feeds can also contain naphthenic acids and/or functional groups which are not acidic but which are converted to organic acids at the temperatures encountered in the extract recovery section.
Thus, the impurities present in NMP recovered by distillation from lube oil distillate extraction processes are different from those present in extraction solvents utilized in aromatics recovery processes in the chemical industry.
It would be advantageous to the extraction process if these impurities and contaminants could be removed from the recovered NMP so as to essentially eliminate this detrimental effect on the extraction plant.